1. Field of the Invention
This invention relates to a method of improving the dimensional and thermal stability of a fibrous web containing substrate and more particularly, to a method wherein the fibrous web is impregnated with a polymer resin cured to a state or stage of full cure and then coated with a polymer resin cured to a state or stage of partial cure.
2. Description of the Prior Art
Printed circuit boards as used in the electronic industry usually consist of a synthetic resin laminate base which is coated on one or both sides with a copper film. The copper acts as the conductor and can be replaced by other conductors and can be omitted in portions of the circuit where conductivity is not desired.
Most of the base materials heretofore in use consisted of laminates having a fibrous web internal skeleton into which polymeric material was impregnated and cured. Such materials were adequate when the circuit boards made therefrom were used for ordinary household goods such as radios and hearing aids. But, when as at present, it is desired to use such printed circuit techniques in precision electronic devices and components, many problems arise calling for particularly stable materials that will assure the reliability of these devices and components during operation under the rigors imposed by the mode and envirooment of use.
Most previously used materials exhibit the following shortcomings:
1. Their electrical properties change beyond tolerable limits with changes in ambient temperature and humidity.
2. Their dimensional stability varies with humidity, temperature and time, showing considerable warping or shrinkage.
3. They generally show poor machinability unles heavily plasticized, which thereby causes loss of important electrical characteristics such as dielectric qualities.
4. Their chemical resistance to many commonly used processing chemicals is limited, requiring that dip-soldering, plating, etching and similar operations be limited with respect to useful materials or mode of treatment.
Further, under many conditions of industrial use, the chemical, dimensional and electrical qualities of the previously used materials are destroyed or seriously compromised by ambient conditions such as extremes of heat, cold, humidity, bacteria and molds, oils, dusts and variations or combinations of such ambient conditions.
Fibrous web laminates useful for electric circuits and suitable for mass production of industrial and electronic apparatus should have excellent physical, chemical and electrical properties and they should be readily machinable and should be easily manufactured without cumbersome processing steps or equipment.
An adequate fibrous web skeletal laminate for printed circuits should have good dimensional stability and good thermal stability, as for example to soldering operations. A minimum dimensional and thermal stability for printed circuit boards is exhibited by shrinkage or growth of 1.0 mil/inch or less after exposure to a temperature of 121.degree.C for 72 hours.
Improving dimensional stability and thermal stability with a fibrous web polymer laminate for printed circuits by maximizing the cure of the particular polymer system leads to problems of adhesion of a metallic conductive pattern thereto. For printed circuit manufacture a minimum peel strength of at least 5 lbs./linear inch is required. Such a peel strength is normally not possible with most fully cured polymer laminates such as epoxies, polyesters, polyimides, etc., which are metallized by additive electroless metal techniques.
Therefore, a method of improving the dimensional and thermal stability of a fibrous web containing substrate without diminishing its usefulness as a printed circuit material is desired. Also desired is a laminate for printed circuit board manufacture having good dimensional and thermal stability, as well as good adhesive properties with respect to metal deposited thereon.